I purchased one piece of MDF from the local timber merchant (you can pick up similar from Wickes) and cut it to the size of the bed frame, cut out a large arch and then nailed it to the bed.
We’d be interested to see how the MDF cut parts hold up over time compared to real LEGO bricks made in ABS, which seem to last forever. This isn’t the first make-your-own-LEGO hack we’ve see ...